The present invention relates to a laminated packaging material which is especially suitable for heat-sealable packages for liquid food, and which comprises a core of paper or cardboard, and oxygen and aroma proof barrier layers of ethylene-vinyl alcohol copolymer (EVOH) and polyamide (PA). Further, the invention relates to a manufacturing method of the same and to a sealed package for liquid food, manufactured of the material, e.g. a milk or juice carton.
Laminated packaging materials used for liquid food packages and based on paper or cardboard are usually provided with one or more barrier layers the purpose of which is to prevent the contamination of the packed product due to oxidation, and to preserve the product""s taste and vitamines contained by it. Of polymeric materials, EVOH is known to have excellent barrier properties, but also polyamide has been used in liquid package cardbdards, because of its good barrier properties.
An example of the use of polyamide in a liquid package cardboard is described in the publication print FI 86610 and, correspondingly, the use of EVOH is described, for example, in the publication print FI 89567. In accordance with both the prints, the packaging material has been laminated on both sides with an external heat-sealing layer of LDPE (low-density polyethylene) so that the cardboard layer and the barrier layer of polyamide or EVOH is interposed between the heat-sealing layers. In the patent publication U.S. Pat. No. 4,701,360, there are known laminated packaging materials for liquid packages which may, besides an EVOH layer, include another oxygen proof barrier layer e.g. of polyamide. According to the publication, the EVOH layer is situated on the surface of the packaging material so that it also acts as a heat-sealing layer. In this case, EVOH is in contact with the packed liquid, which is a poorer solution with regard to the barrier properties of the material than the one in which the EVOH layer would be protected by a separate heat-sealing layer.
In the EP publication 0 318 771 there is disclosed a packaging laminate comprising EVOH and polyamide layers in combination with an aluminium foil. According to this publication the aluminium foil constitutes the main gas barrier of the laminate and this barrier has been enhanced by an EVOH layer in order to compensate for small cracks that might occur in the foil. An optional polyamide (nylon) layer may be included to absorb moisture penetrating through said cracks. According to the teachings this layer is in general not necessary but preferred when the laminate is in danger of being subjected to strong external stresses, that is, in danger of developing cracks. If a polyamide layer is included it would have a thickness of 15-25 xcexcm, preferably 20 xcexcm. The aluminium foil in such a laminate forms an excellent oxygen barrier, but a drawback is the high cost of the foil.
The problem with EVOH in cardboards for liquid packages is that it is mechanically weak. For example, upon manufacture of carton-shaped packages, the material has to be folded, which easily causes cracks in the EVOH layer weakening its barrier properties. In addition, the EVOH layer tends to form holes in the area of its joints when the package is heat-sealed. This is caused by steam pressure which tries to escape from the cardboard layer because of heat and which EVOH cannot retain. In this respect, polyamide is a better material than EVOH, because it is less vulnerable to cracking upon folding, and it can also absorb moisture released from the cardboard. Because the barrier properties of polyamide are not egual to those of EVOH, it cannot as such replace EVOH, but it has been tried to solve these problems by combining EVOH and polyamide in a packaging material. Thus, the publication print FI 96752 discloses a method for solving the problem related with the formation of holes by a suitable choice of material for the barrier layer, one alternative being a compound of EVOH and polyamide. The purpose is to retain the viscosity and strength of the barrier layer material in the heat-sealing temperature. Also the patent publication U.S. Pat No. 4,977,004 disclosing a food package cardboard comprising two separate EVOH layers recommends the compounding of EVOH and polyamide to improve the viscosity of the layer. However, a drawback of these solutions is that the barrier properties of EVOH suffer due to the compounded polyamide. The best way to make the material oxygen and aroma proof is to include a continuous substantially clean EVOH layer in the packaging material. In addition, it has been discovered that the compound of EVOH and polyamide is more difficult to extrude than pure polymeric materials, mainly due to the tendency of the compound to gel.
It is the object of the present invention to provide a solution for combining EVOH and polyamide especially in a laminated packaging material for liquid food without the above-mentioned drawbacks. Thus, it is the object of the invention to prevent the formation of holes in the layer containing EVOH, and, simultaneously, to keep the amount of polymer in the layers as low as possible. The laminated packaging material of the invention is characterized in that the layer of ethylenevinyl alcohol copolymer (EVOH) and that of polyamide (PA) are joined together without a layer of adhesive material inbetween, and that the material comprises a polymeric heat-sealing layer on both sides so that the core of paper or cardboard and the said barrier layers are interposed between the heat-sealing layers.
Thus, the present inventin is based on the observation that the excellent barrier properties of EVOH and the viscosity and absence of holes in polyamide may be combined, without deteriorating either property, by joining the barrier layers of the said materials directly to each other without an intermediate adhesive layer, such as a Surlyn layer, which has typically been used to adhere different layers in liquid package cardboards. The good adhesion of the EVOH and polyamide layers observed in accordance with the present invention is supposedly due partly to a chemical reaction between polymers and partly to polarity, i.e. the setting of the negatively charged EVOH and the positively charged polyamide. The absence of the adhesive layer reduces the number of layers compared to other conventional multi-layer structures, which means the extrusion process becomes simpler and causes a saving in materials. The saving in materials and the lighter structure may also be achieved because the necessary barrier properties and the absence of holes may be provided with smaller amount of EVOH and polyamide than before. In general the laminated polymeric layers of cardboard potentially have a negative effect on the smell and taste of a product, and, for this reason, the invention serves the general goal to keep the amount of polymers in a packaging material as low as possible. The packaging material of the present invention can also endure the sterilization processes included in aseptic packaging.
In accordance with the present invention, it is characteristic of EVOH and polyamide that they can be joined together or to a core of paper or cardboard in the packaging material without an intermediate adhesive. An adhesive layer may instead be necessary between the barrier layers and the heat-sealing layer on the material surface. The heat-sealing layers on both sides of the material may advantageously be made of polyolefine, such as LDPE. For example surlyn or LDPE modified with maleic anhydride, may be used as adhesive.
In case the barrier layers are arranged in the packaging material so that the polyamide layer is closer to the paper or cardboard core, the polyamide protects the EVOH layer by receiving and absorbing moisture released from the cardboard layer upon heat-sealing. The EVOH layer is protected against the formation of holes, and it retains its impermeability to oxygen and aromas in the joints. However, it is also possible to place the EVOH layer against the paper or cardboard core, which is less advantagous for the tightness of the layers, but which, on the other hand, has the advantage of the excellent adhesion of EVOH to paper or cardboard. In this case, no limits have to be set for the quality of paper or cardboard used in the packaging material.
For the barrier properties, the sufficient amount of polyamide in the packaging material may be about 1-8 g/m2, preferably 2-5 g/m2, and the amount of EVOH likewise about 1-8 g/m2, preferably about 2-5 g/m2. In known packaging materials with a barrier layer either of EVOH or polyamide, the typical amounts have been about 6-8 EVOH/m2 and 8-10 g PA/m2. Thus, the separate, opposite EVOH and polymer layers of the present invention do not increase the amount of polymer required by the barrier layers in the packaging material, but, rather on the contrary, they may be able to reduce it. The amount of polymer in the heat-sealing layer may be about 5-50 g/m2 on both sides of the material.
The method of the present invention for manufacturing the said laminated packaging material, in which polymeric laminate layers are placed on a core of paper or cardboard by extrusion, is characterized in that EVOH and polyamide are placed against each other as molten webs, whereafter they are forwarded to the core in a combined web. The barrier materials then have time to become effectively amalgamated before they meet the paper or cardboard, and, thereafter, solidify.
A liquid food package of the present invention, which is manufactured from the said packaging material by heat-sealing, is typically a sealed carton mainly of rectangular shape, and it may contain, for example, milk, cream, sour milk, yoghurt, juice, or wine. However, it may be mentioned that the polymeric barrier layers do not act as an oxygen or aroma barrier in an unopened package only, but they also help to preserve the product even after the package has been opened.